When a sample is scanned by irradiating it with electrons to detect secondary charged particles emitted from the sample, the structure of the surface of the sample can be observed. A device used for such observation is called a scanning electron microscope (hereinafter abbreviated as the SEM). Meanwhile, the structure of the sample surface can also be observed by using an ion beam. A device used for such observation is called a scanning ion microscope (hereinafter abbreviated as the SIM). The ion beam is more sensitive to information about the sample surface than an electron beam. The electron beam is a wave of electrons and inevitably aberrated due to a diffraction effect. The ion beam, on the other hand, is insignificantly aberrated due to the diffraction effect because ions are heavier than electrons.
A gas field ion source is as suitable ion source for an ion microscope. The gas field ion source is capable of generating an ion beam having a narrow energy width. Further, as an ion generation source is small in size, it can generate a fine ion beam.
To use the ion microscope for observing a sample at a high signal-to-noise ratio, it is necessary to obtain an ion beam having a high current density. To obtain such an ion beam, it is necessary to increase an ion radiation angle current density of a field ion source. The ion radiation angle current density can be increased by increasing the molecular density of an ion material gas (ionization gas) in the vicinity of an emitter tip.
The molecular density of a gas per unit pressure is in inverse proportion to the temperature of the gas. Therefore, the molecular density of the ionization gas in the vicinity of the emitter tip can be increased by cooling the emitter tip to an ultra-low temperature for the purpose of lowering the temperature of the gas in the vicinity of the emitter tip.
The molecular density of the ionization gas in the vicinity of the emitter tip can also be increased by increasing the pressure of the ionization gas in the vicinity of the emitter tip. Under normal conditions, the pressure of the ionization gas in the vicinity of the emitter tip is approximately 10−2 to 10 Pa. If the pressure of the ionization gas is further increased to increase the pressure of the ion material gas to approximately 1 Pa or higher, the ion beam collides with a neutral gas and becomes neutralized, thereby decreasing an ion current.
Moreover, when the number of gas molecules in the field ion source is increased by raising the pressure of the ionization gas, the gas molecules whose temperature rises when they collide with a wall of a high-temperature vacuum vessel collide with the emitter tip at an increased frequency. Thus, the temperature of the emitter tip rises to decrease the ion current. To avoid such a decrease in the ion current, the field ion source has a gas ionization chamber that mechanically surrounds the emitter tip.
In an example disclosed in Patent Literature 1, the gas ionization chamber surrounds the emitter tip by using an ion extraction electrode while the ion extraction electrode is provided with an ionization gas introduction port.